(a) Field of the Invention
The present invention relates to an apparatus and method for controlling a speed change of a hybrid vehicle. More particularly, the present invention relates to an apparatus for controlling a speed change of a hybrid vehicle, in which shifting is performed in a constant power region of a motor for adjusting a regenerative braking torque to be substantially uniform.
(b) Description of the Related Art
A hybrid vehicle uses two or more power sources, in general, an engine and a motor. Since the hybrid vehicle uses two or more power sources of the engine and the motor, the hybrid vehicle may have a variety of structures. The motor provided to the hybrid vehicle operates to supplement engine power during acceleration or hill climbing (e.g., uphill driving). In addition, the motor provided to the hybrid vehicle is operated as a generator during braking of the vehicle to generate braking force by converting kinetic energy generated during the braking to electrical energy. Thus, the converted electrical energy is charged to a battery.
A system for converting the kinetic energy generated during the braking to electrical energy and recovering the electrical energy thus recovered is called a regenerative braking system. FIGS. 1 (a) and (b) are exemplary graphs showing characteristics of a motor applied to a general hybrid vehicle, respectively. Referring to FIGS. 1 (a) and (b), the motor applied to the hybrid vehicle has a substantially constant torque region and a substantially constant power region divided based on a rotation speed of the motor.
Referring to FIG. 1 (a), when the rotation speed of the motor is less than a threshold speed, though the torque of the motor is maintained substantially constant, when the rotation speed of the motor is greater than the threshold speed, the torque of the motor may reduce exponentially. In particular, a section in which the torque of the motor is maintained substantially constant is called a constant torque section. Referring to FIG. 1 (b), when the rotation speed of the motor is less than the threshold speed, although the power of the motor increases linearly, when the rotation speed of the motor is greater than the threshold speed, the power of the motor is maintained substantially constant. In particular, a section in which the power of the motor is maintained substantially constant is called a constant power section.
Further, referring to FIG. 2, a total braking torque of the hybrid vehicle (see FIG. 2 (c)) is a sum of a hydraulic pressure braking torque (see FIG. 2 (b)) from a hydraulic pressure braking system and the regenerative braking torque (see FIG. 2 (a)). In particular, when the regenerative braking torque is reduced, it is required to meet a required braking torque by increasing the hydraulic pressure braking torque. However, a general hydraulic pressure braking system has a slower response speed than that of the motor. Therefore, since a time point at which the braking torque of the hydraulic braking system supplements the regenerative braking torque of the motor is delayed at a section in which the regenerative braking torque of the motor rapidly changes, a driver may feel such a sense of difference. Particularly, the sharp or rapid change of the regenerative braking torque occurs distinctively at a threshold speed where the constant power section changes to the constant torque section during a speed reduction of the hybrid vehicle.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.